An Mk2 version of Music Thing Modular’s Spring reverb module went on sale this weekend. These are some notes about what the module does, how it was designed, and how to get the most out of it.

Spring reverb is a strange, unpredictable thing. It’s a real electromechanical effect; boingy metal springs being jiggled by a little coil, with the jiggling captured by another coil at the far end. Compared with smooth, reliable software, it can sometimes be frustrating and fiddly to use, but nothing else sounds the same.

In 1935, Laurens Hammond was developing and selling his Hammond Organ. He’d brought the church organ into the living room, but found they sounded a bit underwhelming without the reverberation of the hall. He discovered that Bell Labs had created an artificial delay device to simulate long-distance calls, using wire and springs. He re-purposed and tweaked the technology to create the first artificial reverberation unit. At first, they were huge.

By 1960, spring reverb units had shrunk enough to fit into guitar amplifiers. In the early sixties, both Moog and Buchla modulars had reverb modules. In 1969 The EMS VCS3 had a spring reverb inside the case, as did the Arp 2600 (1971) and the Buchla Music Easel from 1973.

Traditional guitar reverbs are tuned to the sound of the guitar and the amplifier it is played through. This circuit is designed to be relatively clean and hi-fi, with a lot of bass response.

Spring reverb tanks come in many flavours. If you’re buying a new tank for this module, I’d recommend one of these fairly common models: An Accutronics 9EB2C1B A big 17” long reverb tank with six springs or an Accutronics 8EB2C1B, which is a smaller 10” version with three springs. This is the tank from a Fender Blues Junior amp, so is very common. Other makes with the same number (i.e. MOD 8EB2C1B or Ruby 3EB2C1B) should also work.

The Tested spring specs are: Input: 150Ω to 800Ω, output: 2,250Ω to 2,575Ω, with Insulated input, Grounded output. Beyond this range, you may need to do some research or experimentation.

The Brick is a curious patented device designed in 2007 by guitar effects legend Brian Neunaber. Inside the epoxy block are three PT2399 chips. These are cheap digital delay chips designed for karaoke machines. The brick uses them to create short reverb-like delays. I used to have a Dynacord VRS23, a rackmount analog delay from the ’80s. It had a ‘reverb’ mode that sounded very like one of these bricks. Only BTDR-2H bricks will work (you can also use BTDR-2V bricks, but they’ll poke out the back of the module some way).

Spring Reverb Mk2 diagram: Click to enlarge

What’s new? Feedback

This new version adds a feedback control and a feedback inversion switch, to add anything from a subtle ‘glow’ to screaming howlround.

Mixing just two sine waves, phase has a huge impact on the output level. With the multiple signals in a complicated sound running through a spring reverb, the effect is chaotic (image via Daniel A. Russell of Penn State)

Reverb is a really complex audio signal — multiple copies of the original sound (including the original itself), delayed and filtered by different amounts. That’s why phase is important. Signals that are in phase get louder, signals that are out of phase get quieter.

The feedback switch inverts the signal as it feeds back into itself, or turns feedback off. With a spring, the difference is hard to describe but easy to hear. One version might be brighter, the other duller. In one setting, it might feed back easily, the other more reluctantly. With a brick, runaway feedback seems to only happen in one position (the middle setting) while the bottom setting feels more like a short delay.

In both cases, results will be unpredictable. As ever, the input signal has a huge impact on the output. Sometimes, feedback is immediate and uncontrollable, other times it never comes. The louder the signal, the more likely it is to tip into feedback, but — again— it’s not that simple.

This is one reason why on this module feedback isn’t CV controlled — it normally seems to need a human ear and hand as part of the circuit. That said, it’s easy to patch CV-controlled feedback by putting a VCA in the feedback loop. It’s all about experimentation — flick, turn, listen, keep your hand on the controls to stop runaway noise (or encourage it). The Tilt control is also really important in shaping the feedback.

The feedback input can also be used as a second input to the reverb, with the level set by the feedback control — making it a simple two-channel mixer, with or without reverb.

What’s new: an interruptible Feedback Loop

Patch anything between Wet and Feedback to disrupt the feedback path; bandpass filters are particularly effective, delays or waveshapers are fun. A phaser would be interesting.

What’s new: Mix Expander

The main reverb module has phono sockets on the back, so you can put a reverb tank inside your modular case.

Often, you might want to mount the spring tank outside the case; for small cases, or for access to the springs — stroking them or muting them or ‘preparing’ them in some way.

This little 3hp expander makes that simple. It’s reversible, so you can arrange the cables how you like .

The expander also has a simple mix pot to blend the spring and brick reverbs together — this is a really nice effect, particularly when using feedback.

Weird trick

If you use the expander with a brick, but without a spring connected to the front or back of the module, the spring drive circuit will overdrive massively (the spring pickup is part of the feedback loop of an op amp, so without it you get open loop gain). Fading across to spring and playing with feedback and EQ can get some really crunchy, squeaky extreme distortion that can be quite controllable with the blend control.

Not new but useful: Crossfade Input

The X-Fade input turns the module into a voltage controlled crossfader (without any reverb). The circuit uses a vactrol, so has a little bit of lag (or bounce). Both inputs must be used — so you can’t really crossfade into silence and have it work like a VCA. It’s a very simple circuit, and you may get a little bleed (clean sounds still audible when you have 100% wet selected).

The crossfader is nice for a classic Buchla trick; patch a sine into the main input, and a square or sawtooth from the same oscillator into the X-Fade input, and you get something that feels like a low pass filter. Try ‘pinging’ it with a trigger or short gate into the CV input and you should be able to hear the vactrol decay. Here’s a demo of crossfade mode on the Mk1 Spring Reverb.

Demo Videos

Here’s the section on the Spring Reverb from my 2017 Superbooth preview:

How to get a warm, rich sound from spring reverb

I’m very happy with how this circuit sounds, particularly in this ancient demo:

It’s not always easy to coax such smooth tones from a spring reverb. Don’t assume there is a fault with your module if you don’t immediately get this kind of sound — I made that mistake a few times on breadboard.

What you get out depends on what you put in. By far the biggest factor in the tone of the reverb is — obviously — the tone being fed into it. Software reverbs can put the same smooth mushy sound over whatever your run into them. This isn’t like that at all. Remember that it’s a physical system moving about, driven by magnets and electricity and springs.

I’ve found that smooth, clean, FM-ed sinewaves work very well with the springs. On a ‘scope, the output looks similar to the input, but more complicated. On the other hand, square waves try to bash the springs about. There’s no way that a magnet can move a spring in anything like a square wave, so the sound coming out will sound little like a square wave. On a ‘scope, it’s more like a filter or waveshaper.

Choice of spring tank is not absolutely critical. Big long six spring tanks will sound bigger than small tanks, but I’ve found that — at least with my limited selection — it is less important than the other factors in this list. I wouldn’t recommend the tiny plastic-boxed new reverb that Belton have developed. You can always use EQ to compensate for the choice of tank to some extent.

Hum is caused (and can often be cured) by the exact position of the tank. A reverb tank is not unlike a big, highly amplified single coil guitar pickup. It will pick up 50/60hz mains hum, particularly from transformers but probably also from dimmer switches or fluorescent tubes. That hum can often be stopped completely by moving the tank away from the source or — often more practically — by just changing the position of the tank. Try shifting it through 90 degrees — on whichever axis is practical — and often the hum will drop. If you want to mount your tank inside a case, experiment with the exact position to find the quietest spot before fixing anything.

Noise (hiss, white noise, not hum) is caused by the high-gain op amp in the recovery section. I’ve tried to reduce it as far as possible by using low-impedance resistors. I also tried two lower-gain op amps in series, which seriously increased the noise. It may be that using other dual op-amps could reduce this noise even further. Or just think of it as tape hiss, adding mojo.

Spring reverb is (once more) chaotic and hard to predict. Spring reverb is a strange, gnarly, physical effect. When I tried to calibrate and measure this circuit, I’d find that a sine wave at, for example, 400hz, might barely excite the springs at all, while a sine at 405hz would bust them into fuzzy overdrive.